Coherent light generators – Raman laser
Reexamination Certificate
1999-09-21
2003-05-20
Leung, Quyen (Department: 2828)
Coherent light generators
Raman laser
C372S006000, C372S094000, C372S099000
Reexamination Certificate
active
06567430
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to optical oscillators and amplifiers used in fiber-optics for telecommunications, cable television and other fiber-optics applications. More particularly, the invention relates to optical oscillators and amplifiers based on Raman gain in a fiber that provides for a particularly simple implementation based on intracavity use of periodic transmission filters.
BACKGROUND ART
Optical amplifiers are one of the key enabling technologies for exploiting the bandwidth available in optical fibers. For example, optical amplifiers can be used to compensate for loss in fiber-optic transmission. Loss refers to the fact that the signal attenuates as it travels in a fiber due to intrinsic scattering, absorption and other extrinsic effects such as defects. Examples of optical amplifiers include erbium-doped fiber amplifiers (EDFAs) and Raman amplifiers. A key feature of optical amplifiers is that they be low-noise and broadband, thereby permitting wavelength-division-multiplexed (WDM) systems.
There are two main low-loss telecommunications windows in optical fibers at wavelengths of 1.3 and 1.55 microns. EDFAs have become the workhorse of the optical amplifier field, but they only operate in the 1.55 micron window. Raman amplifiers have the advantage that they can operate in both optical communication windows, and, in fact, over the entire transparency window of optical fibers. Also, Raman gain increases system reliability since there is no excess loss in the absence of pump power. Moreover, Raman-based amplifiers are fully compatible with fiber systems since they are all-fiber devices.
Stimulated Raman scattering amplifiers work on an entirely different principle than EDFAs. Stimulated Raman scattering amplifiers are based on nonlinear polarization of the dielectric silica host, whereas EDFAs are based on the doping of glass fibers with rare earth ions. Signal amplification in Raman amplifiers is due to stimulated scattering accompanied by the excitation of molecules into a vibrational state. In contrast, signal amplification in EDFAs is due to stimulated emission accompanied by relaxation of the excited ions to the ground state. Thus Raman amplifiers and erbium-doped amplifiers work on entirely different physical principles.
The nonlinear polarization in Raman amplifiers is third order in electric field strength, resulting in a nonlinear index of refraction and gain that are both proportional to the instantaneous pump intensity. In contrast, the medium polarization is linear in the EDFA. Also, whereas EDFAs have an upper state lifetime of about 10 msec, Raman amplifiers have a virtually instantaneous response.
The theoretical noise-figure contribution from signal-spontaneous beating for Raman amplifiers has been shown to be 3 dB. However, systems tests of Raman amplifiers have uncovered other sources of noise that generally are not important in EDFAs (c.f. A. E. White and S. G. Grubb, “Optical Fiber Components and Devices,” Ch. 7
in Optical Fiber Telecommunications IIIB
, eds. I. P Kaminow and T. L. Koch, Academic Press, 1997). The first source is the coupling of intensity fluctuations from the pump light to the signal. The fundamental cause of this noise is the lack of a long upper-state lifetime to buffer the Raman gain from fluctuations in the pump intensity. It has been shown that when a counter-propagating amplifier geometry is used, the transit time of the amplifier can be used to average gain fluctuations due to the pump. Second, double Rayleigh can also give significant contributions to the noise figure of Raman amplifiers because of the long lengths of fiber used. However, limiting the fiber lengths used and constructing multistage amplifiers can control the noise figure of the amplifier.
Several Raman laser and amplifier cavity designs exist as prior-art, but they are not very appropriate for broadband amplification of WDM systems. S. G. Grubb and A. J. Stentz (Laser Focus World, pp. 127-134, February 1996; also U.S. Pat. No. 5,323,404) have described a linear cavity that uses a series of gratings to define the end mirrors. However, the bandwidth of the gratings is sufficiently restrictive that the cavity can operate over only about 2 nm, which is inadequate for WDM applications.
As an improvement, Grubb, et al. (U.S. Pat. No. 5,623,508) also describe a ring cavity that uses an intra-cavity isolator to reduce double Rayleigh scattering and uses a counter-propagating pump to avoid pump fluctuations from coupling to the signal channel. The ring cavity design, however, is substantially more complicated, and, since it also employs gratings, it is also narrow band.
Rather than using gratings, Chernikov, et al. (Electronics Letters, Vol. 31, pp. 472-473, March 1995), use wavelength selective couplers in their Raman cavity design. Whereas their original 1995 design uses five couplers, a simpler configuration using only two couplers is described later (Electronics Letters, Vol. 34, pp. 680-681, Apr. 2, 1998). The couplers used are fused fiber couplers that couple over certain Raman orders into a ring cavity while passing other Raman cascade orders onto an end mirror. By using these broader band devices they achieve a bandwidth between 6-10 nm. However, the couplers may be difficult to manufacture, are somewhat inefficient in that they do not completely couple over or pass through any of the Raman orders, and there are no means in the cavity for rejection or dampening of the double Rayleigh scattering.
Broader band designs of Raman cavities have also been disclosed. As described in U.S. Pat. No. 5,778,014, there are several advantages of the Sagnac Raman amplifier and laser designs over those based on gratings or wavelength-selective couplers. First, the Sagnac cavity is a simple, easily manufactured, all-fiber cavity that should reduce the cost of assembly and increase the device reliability. Second, the passive cavity Sagnac interferometer design has a noise dampening property during the cascaded amplification process, thereby leading to improved noise performance and stability. Third, the broadband cavity design and components should lead to a wider gain bandwidth (in excess of 10 nm) for WDM applications. However, since there is no wavelength control within the cavity, changes in pump power may lead to fluctuations in the output wavelength. Also, the Sagnac requires use of polarization controllers, unless the cavity is made of all polarization-maintaining components. Finally, the Sagnac may have a lower efficiency than linear grating-based cavities since the pump light is split along the two directions of the Sagnac.
An alternate cavity based on a circulator loop cavity and the use of chirped fiber gratings has also been described in the above-noted U.S. patent application No. 60/120,408 Feb. 12, 1999. The chirped gratings can also be composed of a series of gratings. The reflection band of each band is slightly shifted in frequency. The circulator loop design permits a strictly counter-propagating pump for the signal wavelength, and the chirped fiber gratings permit wavelength control while still allowing for broadband behavior for each Raman cascade order. Hence, the circulator loop design can be low-noise and broadband at the same time. However, the design requires circulators or isolators that provide a sufficient amount of isolation over several Raman orders. Such broadband devices are not available as yet, although they could potentially be composed of a cascade of circulators or isolators operating at each Raman order.
Consequently, there is a need for a Raman oscillator or amplifier would have the best features of all of these designs. The desired attributes for the cavity include:
high-efficiency and low intracavity loss, such as in the grating-based designs;
low-noise performance by using strictly counter-propagating pump and signals, such as in the grating-based ring designs;
broadband designs, such as the Sagnac Raman cavity;
stable wavelength operation, such as in the circulator loop cavity with chirped gratings; an
Freeman Michael J.
Harris Hayden H.
Islam Mohammed N.
Baker & Botts L.L.P.
Rodriguez Armando
Xtera Communications Inc.
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